At present, in a packet switching communication system with a high capacity and a high throughput, information transmission between a line card and a switching network generally adopts a queue back-pressure mechanism, that is, the switching network detects a data sending condition at each output port, and when congestion occurs at the output port, the switching network generates back-pressure information to inform all the line cards to stop sending data to the output port. If two data streams of different service levels respectively come from two different line cards, the sending of the two data streams is stopped at the same time, and a data stream of a higher service level may also not be sent preferentially. Therefore, how to ensure the effective transmission of the data stream of the higher service level becomes a principal technical issue to be solved in the packet switching communication.